Reference interval for whole blood ionized magnesium in a healthy population and the stability of ionized magnesium under varied laboratory conditions.

OBJECTIVE: To establish a reference interval for ionized magnesium (iMg), to evaluate the stability of whole blood iMg under varied laboratory storage conditions, and to study the correlation between total and iMg in healthy volunteers and in an intensive care unit (ICU) population.
METHODS: Blood specimens were collected anaerobically from 125 healthy volunteers and 200 ICU patients into tubes containing lithium heparin, transported to the laboratory on ice, stored at 4 degrees C and analyzed within 2 h on a NOVA 8 Electrolyte Analyzer. Additional specimens were stored under different conditions before analysis to assess the stability of iMg.
RESULTS: In healthy volunteers, the mean whole blood iMg level was 0.52 mmol/L with a range of 0.44 to 0.59 mmol/L. The iMg/total serum Mg ratio was at 0.60 (0.50-0.69). Regression analysis of iMg vs total Mg produced a correlation coefficient (r) of 0.48 (p < 0.0001). Ionized Mg levels were comparable between males and females (0.52 +/- 0.04 vs 0.51 +/- 0.03, p = 0.38). In the ICU group, a wider range of iMg results was observed (0.35-0.78 mmol/L) and the correlation between iMg and total Mg was r = 0.71 (p < 0.0001). Storage of whole blood specimens in uncapped tubes at room temperature resulted in a small, but significant, decrease in iMg over a 6-h period. Little change was seen in capped tubes stored either at room temperature or at 4 degrees C. over 6 h, or at 4 degrees C. over 5 days.
CONCLUSIONS: Using the NOVA 8 Electrolyte Analyzer, we established a reference interval for whole blood iMg in a healthy Canadian population. The correlation between ionized Mg and total Mg was weak, but statistically significant. Ionized Mg levels in lithium heparin tubes were stable over 5 days when specimens were stored at 4 degrees C, suggesting that specimens may be stored at least overnight prior to processing.